Field of the Invention
The present disclosure relates to control of an image forming apparatus, and particularly to control of an electrophotographic image forming apparatus.
Description of the Related Art
Electrophotographic image forming apparatuses such as multi-function peripheral, copier, and printer have become widespread. The electrophotographic image forming apparatus performs, in a printing process, the steps of: uniformly charging an image carrier while rotationally driving the image carrier; forming an electrostatic latent image by exposing the image carrier; causing toner to stick to the electrostatic latent image on the image carrier; transferring the toner image on the image carrier to a printed matter; and removing toner remaining on the image carrier.
In the step of removing toner remaining on the image carrier, a blade cleaning system, for example, is used. According to the blade cleaning system, a flat-plate-like cleaning blade made of an elastic body is brought into contact with the surface of the image carrier, and the cleaning blade removes toner remaining on the image carrier.
To meet the recent demand for enhanced image quality, the size of toner particles has been decreased. As a method of decreasing the size of toner particles, a polymerization method such as emulsion polymerization or suspension polymerization for example may be used. As the size of toner particles decreases, adhesion between toner particles and the image carrier increases, which makes it difficult to remove toner remaining on the image carrier.
The so-called polymerized toner manufactured by the polymerization method is substantially spherical, and therefore rolls on the image carrier. Due to this, the toner passes by the cleaning blade, namely the so-called “pass-by” occurs. As a result, a cleaning failure is likely to occur, which makes it still more difficult to remove toner remaining on the image carrier.
A method of solving this “pass-by” issue may be a method which applies onto the image carrier a solid lubricant such as metal salt of fatty acid. According to this method, the image forming apparatus applies the lubricant onto the image carrier to reduce the surface energy on the surface of the image carrier and thereby weaken the adhesion between the toner particles and the image carrier and accordingly remove the toner remaining on the image carrier.
Japanese Laid-Open Patent Publication No. 2014-142472 discloses an image forming apparatus controlling the amount of a lubricant applied onto an image carrier. In the image forming apparatus, the surface of the image carrier is divided into a plurality of regions extending in the direction crossing the rotational direction, to allow the lubricant to be uniformly applied to these regions. In this way, the image forming apparatus lessens wear of the image carrier which is caused by nonuniform application of the lubricant.
By way of example, the method of supplying a lubricant onto an image carrier includes a lubricant application mechanism system and a toner external additive system. In the case of the lubricant application mechanism system, a rotating brush is brought into contact with a solid lubricant which is called lubricant rod, and the lubricant scraped by the brush is supplied onto the surface of the image carrier. In the case of the toner external additive system, a toner containing a lubricant forms a toner image, and the toner itself functions as a lubricant.
In the case where the lubricant application mechanism system is employed, a part of the lubricant may be mixed into a developing unit. In the following, the lubricant mixed into the developing unit is also referred to as “mixed lubricant.” The amount of the mixed lubricant increases or decreases depending on the image portion ratio at the time of image formation. “Image portion ratio” herein refers to the ratio of a toner sticking region, which is a region where toner sticks, to an electrostatic latent image or toner image. In the following, a region where toner sticks in an electrostatic latent image or toner image is also referred to as “image portion.” Further, a region where the toner does not stick in an electrostatic latent image or toner image is also referred to as “background portion.”
In the case where images with a low image portion ratio are successively given, the amount of the mixed lubricant increases. A possible reason for this is as follows. Specifically, in the image portion, the lubricant is covered with toner particles that are electrically stuck to the image carrier by a developing bias. Namely, lubricant particles are blocked by the toner particles. Therefore, the lubricant particles are not directly rubbed by a magnetic brush and almost no lubricant particle is mixed into the developing unit. In contrast, in the background portion, the lubricant is not blocked by toner particles, unlike the lubricant in the image portion. The lubricant is therefore directly rubbed by the magnetic brush and a certain ratio of the lubricant sticks to the magnetic brush and mixes into the developing unit. Accordingly, when images with a low image portion ratio are successively given, there is a high possibility that the lubricant is mixed into the developing unit from the background portion and the amount of the mixed lubricant increases.
On the contrary, when images with a high image portion ratio are successively given, the amount of the lubricant mixed into the developing unit from the background portion decreases. In addition, a certain ratio of the mixed lubricant is discharged from the developing unit as will be described later herein, and the amount of the mixed lubricant accordingly decreases.
In view of the above, a problem is now described that arises in the case where images each having an image portion ratio lower than an average are successively given and in the case where images each having an image portion ratio higher than an average are successively given.
In the case where images with a low image portion ratio are successively given, the amount of the mixed lubricant increases as described above. The lubricant which is mixed into the developing unit is electrically charged to a polarity opposite to the toner due to triboelectric charging resultant from friction with the toner. The lubricant particles stick to a carrier which serves to electrically charge the toner, to thereby hinder the toner from being electrically charged, and the lubricant particles also stick to the toner itself to thereby reduce the charge amount of the toner. When the charge amount of the toner is excessively reduced, a problem that the toner sticks to the background portion (so-called fog), or a problem that the toner spills from the developing unit occurs.
On the contrary, in the case where images with a high image portion ratio are successively given, the amount of the mixed lubricant decreases as described above. The mixed lubricant is discharged little by little from the developing unit during image formation. The mixed lubricant is discharged due to the following two factors.
The first factor is that the lubricant sticking to the toner is discharged together with the toner when the image portion is developed with the toner. The second factor is that the lubricant particles which are electrically charged to the opposite polarity to the toner stick to the background portion and the lubricant particles are moved by an electric field from the developing unit toward the image carrier. As a result, when images with a high image portion ratio are successively given, the amount of the mixed lubricant decreases to become smaller than a normal amount.
The mixed lubricant may also act to reduce the charge amount of the toner. In the case where the image portion ratio is lower than a certain value, an average amount of the mixed lubricant is replaced. Thus, the charge amount of the toner becomes stable. However, in the case where the image portion ratio is higher than a certain value, there arises a problem that the charge amount of the toner is excessively large.
A reason for this problem is as follows. The toner is electrically charged through triboelectric charging resultant from friction with the carrier. The triboelectric charging ability of the toner gradually decreases with time for which the toner stays in the developing unit. This may be caused due to the fact that an external additive contained in the toner is immersed or separated because of stress in the developing unit. The toner is manufactured so that the charge amount is appropriate, on the precondition that printing is done at an average image portion ratio and on the precondition that the toner stays in the developing unit for a certain time.
However, in the case where images with a high image portion ratio are successively given, the toner in the developing unit is successively replaced with newly fed toner, and the time for which the toner stays in the developing unit is extremely shorter than an expected time. As a result, the charge amount of the toner in the developing unit is excessively large. Moreover, the fact that the amount of the mixed lubricant is smaller than normal also promotes the increase of the charge amount of the toner. Further, in the case where replenishment with toner is not timely and the Tc ratio of the developer (the ratio of the toner to the developer) becomes low, the charge amount of the toner particularly increases.
As seen from the above, when images with a high image portion ratio are successively given, the charge amount of the toner becomes excessively large. As a result, the developability is deteriorated and the printing quality is considerably deteriorated.
Therefore, desirably a constant amount of the lubricant is contained in the developing unit. The image forming apparatus disclosed in Japanese Laid-Open Patent Publication No. 2014-142472 aims to apply a uniform amount of the lubricant to the image carrier, rather than to keep constant the amount of the lubricant contained in the developing unit.